Satellite navigation systems, such as the system commonly known as the Global Positioning System (“GPS”), operated by the United States Department of Defense, are well known. Satellite navigation systems are used for determining a precise location almost anywhere on Earth. In particular, GPS can be used by anyone, free of charge, to make such determinations. For this reason, among others, satellite navigation systems are generically referred to hereinafter as GPS.
GPS is divided into three segments: space, control, and user. The space segment comprises the GPS satellite constellation. The control segment comprises ground stations around the world that are responsible for monitoring the flight paths of the GPS satellites, synchronizing the satellites' onboard atomic clocks, and uploading data for transmission by the satellites. The user segment consists of GPS receivers used for both military and civilian applications.
The GPS system uses a satellite constellation of at least 24 active satellites orbiting about 20,000 km above the Earth. Each satellite makes a complete orbit of the Earth every 12 hours. Satellite positions are carefully calculated so that, from any point on the Earth, four or more of the satellites will be in direct line of sight to any location. Each satellite carries four atomic clocks so that the transmission time of the signals is known precisely. The flight paths of the satellites are measured by five monitor stations around the world. A master control station processes their combined observations and sends updates to the satellites through monitor stations. The updates synchronize the atomic clocks on board each satellite to within one microsecond and also adjust the ephemeris of the satellites' internal orbital model to match the observations of the satellites from the ground.
GPS receivers calculate their current position, i.e., latitude, longitude, elevation, and the precise time using the process of trilateration. Trilateration involves measuring the distance to at least four satellites by comparing the satellites' coded time signal transmissions. The receiver calculates the orbit of each satellite based on information encoded in its radio signal and measures the distance to each satellite based on the time delay from when the satellite signal was sent until it was received.
Once the location and distance of each satellite is known, the receiver should theoretically be located at the intersection of four imaginary spheres, one around each satellite, with a radius equal to the time delay between the satellite and the receiver multiplied by the speed of the radio signals.
In practice, GPS calculations are more complex for several reasons. One complication is that GPS receivers do not have atomic clocks, so the precise time is not known when the signals arrive. Fortunately, even the relatively simple clock within the receiver provides an accurate comparison of the timing of the signals from the different satellites. The receiver is able to determine exactly when the signals were received by adjusting its internal clock (and therefore the spheres' radii) so that the spheres intersect near one point.
GPS is used for both military and civilian purposes. The primary military purposes are to allow improved command and control of forces through improved location awareness and to facilitate accurate targeting of smart bombs, cruise missiles, or other munitions. Civilians use GPS for location determination and navigation purposes. Low cost GPS receivers are widely available, combined in a bundle with a PDA or car computer. As such, the GPS system is used as a navigation aid in airplanes, ships and cars. The GPS system can also be used by computer controlled harvesters, mine trucks and other vehicles.
GPS signals can be affected by multipath issues, where radio signals reflect off surrounding terrain such as buildings, canyon walls, hard ground, etc. causing delay in when a signal reaches a receiver. This delay causes inaccuracy in position location. Multipath issues are particularly present in urban environments where a significant amount of obstructions are present. A variety of receiver techniques have been developed to mitigate multipath errors. For long delay multipath, the receiver itself can recognize the wayward signal and discard it. To address shorter delay multipath due to the signals reflecting off the ground, specialized antennas may be used. However, this form of multipath is harder to filter out as it is only slightly delayed as compared to the direct signal, thus causing effects almost indistinguishable from routine fluctuations in atmospheric delay.
Additionally, successful transmission of radio signals, including GPS signals, may be disrupted through the use of jamming technology. Devices making use of such technology, often referred to as “jammers,” can cause navigation and communication problems for radio signal receivers. Radio receivers can be jammed in simple ways, such as by transmitting radio frequency noise in the frequency spectrum in which the receiver operates. More sophisticated jammers use various techniques to alter radio signals like those being sent from satellites. Such techniques may include trying to attack modulation schemes, fooling a receiver into locking onto incorrect radio signals, or mixing signals that inhibit a receiver from demodulating the data of the signal. As used herein, the term “GPS deniers” or “GPS denial devices” shall generally refer collectively to GPS jammers, GPS repeaters (devices that rebroadcast or repeat GPS signals or GPS-like signals, thereby creating confusion for GPS receivers), GPS interrupters (devices that use such techniques as crossover bands and disruptive cancellation to interrupt GPS radio signals from the GPS satellites), and all other devices that are or may be used to disrupt reliable operation of GPS receivers.
GPS is particularly vulnerable to signal disruption because GPS receivers are extremely sensitive. The receivers have to be sensitive to receive relatively weak signals from orbiting satellites. A relatively low-powered jammer, transmitting on the GPS frequency band, can overpower legitimate GPS signals over a wide area—as much as a 100 kilometer circle at just 1 watt radiated power. GPS receivers are so sensitive that there have been documented situations of unintentional jamming. In one such situation three separate jamming signals were being generated by VHF/UHF television antenna preamplifiers. The signals from the preamplifiers were strong enough to completely jam GPS within a one-kilometer radius at sea level.
Because the U.S. military relies heavily on GPS for location determination, the vulnerability of military GPS receivers to being intentionally jammed is particularly critical. GPS jammers and other GPS denial devices may be used to deny signal acquisition and/or confuse a GPS receiver into giving erroneous results. Preventing the denial of GPS to troops in the field is potentially crucial in preventing causalities and carrying out successful military operations. Further exacerbating the problem is the fact that GPS denial devices are difficult to detect and find. Accordingly, a system for detecting and/or locating such jammers is needed.
A related problem pertains to the use of jamming technology against an adversary while minimizing the effects of such use on one's own GPS receivers. More specifically, jamming technology is often difficult to control, affecting friendly GPS receivers the same way as adversarial GPS receivers. If GPS jamming technology is being used anywhere in the vicinity, by a friendly party or an adversarial party, it may not be possible to rely on one's GPS receivers, thus requiring friendly forces to operate with reduced accuracy or, perhaps worse, to operate under the impression that their GPS information is accurate when in fact it is not. Accordingly, a need exists for means to control the use of friendly GPS jamming technology such that the effects of the friendly GPS jammers on one's own GPS receivers are minimized and/or localized.
The foregoing issues highlight a further, over-arching problem, which relates to the need for an easily-deployable system capable of jamming, on a controllable, localized level, enemy GPS receivers while predicting areas in which friendly GPS receivers may be used safely. More particularly, the latter problem requires the ability to identify jamming effects caused by both enemy GPS jammers and friendly GPS jammers.